Optical fiber communication systems have been developed quickly as communication systems having excellent characteristics of high speed modulation and long distance communications. Especially, a coherent optical communication system, in which optical frequency modulation or optical phase modulation is carried out in a transmitting apparatus and the optical signals are detected by optical heterodyne detection in a receiving apparatus, has been attractive among such optical fiber communication systems for its high receiving sensibility in high density frequency division multiplexing which contribute to a large capacity communication as well as a long distance communication. The high density frequency division multiplexing communication method is expected to realize extremely fine motion picture communications, in which it is possible to provide many subscribers with communication services by a relatively small number of stations and a small scale of transmission facilities.
In the coherent optical communication between subscribers and the station by using the high density frequency division multiplexing method, a communication method with a network topology of double star type in which each subscriber is allocated with a predetermined wavelength of optical signals has been reported. In such communication method, it is possible to transmit different kinds of information to different subscribers simultaneously, although it is impossible in a network of tree type topology adopted in CATV now in use, however, a number of wavelength channels corresponding to a number of subscribers are required, so that usage efficiency of wavelengths is not high. In order to improve the usage efficiency, there is a communication method in which a vacant wavelength channel available for communication is selected to be used as a channel in case of each talking occasion without allocating a specific wavelength to each subscriber. Such a communication method is called as Demand Assign frequency division multiplexing method already open to public. This communication method has an advantage in that the number of wavelength channels required in the communication is less than the number of subscribers, so that the usage efficiency of wavelengths increases and station facilities are not required to be large in number and scale.
According to the conventional method for transmission and receipt of coherent light signals, however, there is a disadvantage in that there is no study and report on means in a subscriber for setting a wavelength of a transmitting light source, and adjusting the wavelength to an absolute wavelength for a reference wavelength or a selected wavelength of the station in bilateral communications. In the Demand Assign wavelength division multiplexing communication method, it is required to select a wavelength for communication in each call, so that each subscriber is required to prepare a receiver and a transmitter of wavelength selecting type. A receiver of wavelength selecting type for unilateral communications has been described in detail in Japanese Patent Publication No. KOKAI 1-77325. In this receiver of wavelength selecting type, it is possible to select a desired wavelength channel by controlling a current supplied to a local oscillating light source to be one of memorized currents. However, as explained above, there is no disclosure about transmitter or receiver for bilateral communications.